If Russia armed an enemy of the US with advanced weapons and then helped to plan an attack in which American women and children were killed on a beach in Florida, I would expect America to retaliate. The reverse is happening when America arms Ukraine.
Russia invaded ANOTHER independent country in Europe and seized territory-Crimea in 2014 and now more territory.
So a more accurate example would be if we invaded Canada and Russia gave weapons to the Canadians to defend themselves.